The SA muscle is a flat and wide muscle covering the lateral ribs; it is anatomically divided into three muscle bellies 2. It consists of an upper, middle, and lower muscle belly, each of which contribute to the movement of the scapular bone during upper extremity actions 42. The upper belly of the SA lies parallel to the 1st rib and inserts into the superior angle of the scapula 43. The middle belly of the SA originates from the 2nd, 3rd, and 4th ribs and inserts into the medial scapular border 43. The lower muscle belly of the SA is where the MTrPs frequently exist, originating from the 5th to the 9th ribs and inserting into the inferior angle of the scapula 43,44. The SA muscle is innervated by the long thoracic nerve, which originates from the anterior rami of spinal nerves C5-C7 45. The long thoracic nerve runs superficially over the SA muscle along the anterior axillary line. The SA muscle is mostly involved in upper extremity movements; however, it is the prime stabilizer of the shoulder girdle and acts on shoulder flexion, abduction, and upward rotation 42.
MPS is a chronic pain disorder caused by MTrPs situated at the muscle belly; it has been recognized as the main cause of pain in 85% of patients attending pain clinics 46,47. SA muscle MTrPs may be triggered by muscle strain during excessive running, overloaded weight lifting, or repetitive coughing, especially susceptible to torsional stresses. Another cause of MTrPs initiation in the SA muscle is breast surgery due to cancer or esthetic purposes 48.
Studies have revealed that sarcomere shortening is related to MTrPs etiology, and the shortening is due to an increase in activation of the neuromuscular junction and its over-release of acetylcholine. In addition, a large quantity of calcium released at the sarcoplasmic reticulum over a dysfunctional ryanidine receptor causes prolonged muscle contraction 49. Therefore, to release muscle contraction, BoNT is currently frequently used as an injective agent for MPS 49–51. The primary known therapeutic effects are by releasing muscular contractions and alleviating the vicious pain cycle 52–54. It is also thought that the relief from the muscle tightness and the BoNT inhibit the diffusion of neurotransmitters in the peripheral nerve, avoiding peripheral sensitization 55,56.
In treating MPS, it is critical to locate MTrPs, and electromyography (EMG) may help to verify the presence of MTrPs, as they are thought to have sensitized nerves that spontaneously produce low-amplitude electrical activity 7,57−59. The study by Kuan et al. demonstrated BoNT injection in MTrPs and found that the injection diminished spontaneous electrical activity in MTrPs 60. There are many studies indicating that MTrP pathophysiology appears to be associated with the intramuscular neural arborized areas, also known as identical with MTrPs 7–11. A study by Xie et al. showed that both BoNT injection and lidocaine injection therapy in the intramuscular arborized area substantially lessens the intensity and frequency of pain in patients 18.
Vargas-Schaffer et al. 61 performed trigger point injection in patients with SAMPS and showed its effectiveness in pain control after three months of injective treatment. In 8 patients, the duration of SAMPS was approximately 19 months. With the patient in a lateral decubitus position, the injections were conducted over the midaxillary line on the 5th to the 7th ribs.
Many studies have described frequent SAMPS incidence after breast cancer treatment 48,62−64. Lacomba et al. 48 reported a prevalence rate of 45% for SAMPS one year after breast cancer surgery. Layeeque et al. 21 observed 48 breast cancer patients who underwent mastectomy and observed post-surgical pain; 22 patients were treated with BoNT in the SA and pectoralis major muscle, and 26 control group patients were treated with medication. The BoNT-treated group reported significant pain reduction compared to the pain medication group.
Neuromuscular junctions are the underlying causes of MPS; therefore, injecting BoNT and other injective treatments such as lidocaine, steroids, and normal saline are frequently performed to target the neuromuscular junctions 65–67. Unlike oral medications and lidocaine injections that have short-term effects, the effectiveness of BoNT treatment in MPS has been known to continue for up to 4 months 66,67.
As BoNT acts on the neuromuscular junction, accurate anatomical knowledge of the neuromuscular arborization patterns of the SA muscles is vital for achieving the highest relief with the smallest possible dose of BoNT. Although BoNT procedures are minimally invasive compared to surgical intervention, there is a probability of damaging the nerve trunks that are not present near the neural arborized area. Therefore, precise knowledge of the anatomical features of the SA muscle should be considered. In this study, we carried out Sihler’s staining, which is a whole mount staining procedure that stains myelin sheaths and is effective in tracing the nerve endings without destroying the nerves 14, 38–40,68. The application of Sihler’s staining to the SA muscle will enable an accurate and thorough understanding of the neural distribution.
Moreover, identifying the neural arborization area of the SA muscle is important in diagnosing long thoracic nerve palsy 69. Surface electromyography in the SA muscle is challenging because multiple thin digitations make it difficult to place the electrode for recording 70. When detecting long thoracic nerve palsy, the technical limitations of electromyography are interrupted signals from the neighboring muscles and difficulty with accurate electrode placement since the SA is not a bulky muscle.
At present, there is no standardized injection or EMG points of the SA muscle. In this study, we suggest that electromyography and injective treatments including BoNT, lidocaine, normal saline, steroids should be administered in the three regions in the middle portion, between the 6th to 9th rib portion and the 1st to 5th rib (Fig. 2). Surgeons should be aware that they must inject over the bony rib with patients holding their breath to avoid iatrogenic pneumothorax.